Mass spectrometers are generally used to determine the distribution of the masses of molecules in a sample material. Some mass spectrometers ionize the molecules, and then determine the mass-to-charge ratio of the ionized molecules by analyzing their dynamic behavior in an electro-magnetic field.
The operation of some mass spectrometers includes a loading phase, during which the spectrometer confines the motion of the ionized molecules to a volume inside, for example, a three dimensional (3D) quadrupole ion trap.
The ion trap may include a number of electrodes. The ion trap receives the ionized molecules and confines them by generating, for example, a dynamic electric field via its electrodes. To generate the field, the mass spectrometer may apply to one or more of its ion trap electrodes a time-varying radio frequency (RF) electric signal.
During the loading phase, the mass spectrometer accumulates ions in the ion trap. Further, during the loading phase, some mass spectrometers slow down and trap ions by causing them to collide with neutral gas molecules that exist in the ion trap. The trapped ions thus move in trajectories, or orbits, confined inside the ion trap. The shape of the trajectory or the frequency of the orbit may vary for different ions.
The operation of some mass spectrometers further includes one or more ejection phases. During each ejection phase, the mass spectrometer ejects some of the captured ions from the ion trap towards a detector. During each ejection phase, the mass spectrometer causes at least some of the captured ions to escape the ion trap. The mass spectrometer may cause such escapes by driving the ions to an unstable dynamic state or by driving them to a resonance state. To create the unstable or resonance state, the mass spectrometer manipulates different characteristics of the field that is confining the ions. In each ejection phase, a subset of the ions may reach the unstable state based on characteristics that include their mass-to-charge ratio.
The detector may output a signal proportional to the number of the ejected ions detected in each ejection phase. The distribution of the number of ejected ions detected in an ejection phase indicates a distribution of the mass-to-charge ratios for different ions in the sample. This distribution may include one or more peaks, each corresponding to a group of ions with approximately the same mass-to-charge ratio.
A mass spectrometer's utility depends on a variety of factors. One factor is the spectrometer's sensitivity. The sensitivity is related to the minimal amount of sample that the spectrometer requires for deriving a spectrum with discernible peaks. The sensitivity of a spectrometer is related to the portion of the sample that the spectrometer is able to utilize for detection. The spectrometer cannot utilize all molecules in the sample, as some of the molecules are lost during the operation. For example, during the loading phase, the spectrometer may not capture all of the ions that enter the ion trap. Instead, some of those ions may exit the ion trap without being confined or may exit at an incorrect time. Similarly, during an ejection phase, the spectrometer may not detect all of the ions that are ejected. Thus, the mass spectrometer's sensitivity can be improved by increasing the capture efficiency; that is, the relative number of ions that the ion trap can capture during the loading phase. Similarly, the mass spectrometer's sensitivity can be improved by increasing the ejection efficiency; that is, the relative number of ejected ions that successfully reach the detector.
Another measure of a mass spectrometer's utility is its resolution. The resolution is related to the ability of the mass spectrometer to produce peaks for a given mass to charge that are distinct from adjacent peaks of a different mass to charge. Such resolution may depend on how fine-tuned the spectrometer is during the ejection phases. A spectrometer might not precisely eject ions of a particular mass-to-charge ratio over a short time period, and may thus show a broadened spectral peak for a single mass-to-charge ratio. A spectrometer may, alternatively, eject together ions that have different but close mass-to-charge ratios, and thus combine peaks that represent separate mass-to-charge ratios into a single broadened peak. The mass spectrometer's resolution, thus, improves if the spectrometer can eject each group of ions with the same mass-to-charge ratio over a short time period and separately from other groups with different values of mass-to-charge ratio.